As is known, an AESA antenna, to be able to function properly, requires a calibration system so that it can be calibrated, i.e., so that it can periodically adapt the phase and amplitude of the respective transmit/receive modules (TRMs) in such a way as to achieve the required radiating performance. In particular, in radar systems based upon AESA antennas the term “calibration” is used for describing the measurements and regulations made automatically by the radar systems on the TRMs, especially during start-up, to ensure the required radiating performance.
In this regard, illustrated in FIG. 1 is a block diagram representing a typical architecture of an AESA antenna designated as a whole by 1.
In particular, the AESA antenna 1 includes a beam-forming network or manifold 11, which comprises, at a first end, an input/output port 12 and is connected, at a second end, to a plurality of TRMs 13, each of which is connected to a corresponding radiating element 14.
In detail, the beam-forming network 11 enables:                in transmission, propagation of radiofrequency (RF) signals from the input/output port 12 to the TRMs 13 so that said RF signals will be amplified and phase-shifted by said TRMs 13 and then transmitted by the radiating elements 14; and,        in reception, propagation from the TRMs 13 to the input/output port 12 of RF signals received from the radiating elements 14 and amplified and phase-shifted by said TRMs 13.        
Conveniently the input/output port 12 is connected to transceiving means (not illustrated in FIG. 1) of the AESA antenna 1, which are configured for:                in reception, receiving and processing the RF signals received from the radiating elements 14, amplified and phase-shifted by said TRMs 13 and propagated through the beam-forming network 11 by the TRMs 13 up to the input/output port 12; and,        in transmission, supplying at input on the input/output port 12 the RF signals that the AESA antenna 1 must transmit, which then propagate through the beam-forming network 11 from the input/output port 12 up to the TRMs 13, are amplified and phase-shifted by the TRMs 13, and finally, are transmitted by the radiating elements 14.        
For an AESA antenna to achieve the required radiating performance, it is necessary for there to be for each path among all the elements of the array pre-defined relations of phase and amplitude. The insertion of phase and amplitude of each radiating element depends upon passive components (beam-forming networks, cables, etc.) and active components (TRMs). The aim of the calibration is to regulate the amplification, specifically via a variable attenuator, and the phase of each TRM to obtain the desired distribution of phase and amplitude on the face, i.e., on the surface, of the active array.
Normally, the calibration must be repeated periodically because ageing and/or variations in temperature cause variations in the insertion of phase and amplitude of the TRMs.
In order to carry out calibration, an AESA antenna must be equipped with a calibration system, i.e., additional hardware and software elements that will enable the AESA antenna to measure and regulate insertion of phase and amplitude of each RF path that comprises a TRM (in AESA antennas usually each radiating element is coupled to a respective TRM).
In particular, as regards calibration of an AESA antenna by means of a calibration system it must be possible to inject an RF signal in each RF path of the AESA antenna that comprises a TRM and to measure said RF signal after the TRM, i.e., to measure the amplitude and phase of the RF signals that propagate in each RF path that includes a TRM. Moreover, when the injected RF signal is measured, said RF signal must have a signal-to-noise ratio (SNR) as high as possible so as to obtain accurate measurements.
For example, according to the U.S. patent application No. US2004032365 (A1), in order to calibrate an AESA antenna, an RF signal can be injected using a supplementary RF network that injects the RF signal on each path of the AESA antenna through a coupler, or else using different external antennas to inject the RF signal directly into each radiating element. This second solution requires an amount of additional hardware elements smaller than the first solution, but requires positioning of external antennas outside the structure of the AESA antenna, thus increasing the overall dimensions thereof. This is a disadvantage above all for AESA antennas used in transportable radar systems, where the external dimensions of the AESA antennas must be as small as possible, albeit compatible with the requirements of the antenna (beam aperture, gain, etc.).